The present invention relates to alkaline storage batteries such as nickel-hydrogen storage batteries and nickel-cadmium storage batteries, and to a method for producing the same. Particularly, it relates to alkaline storage batteries the separator incorporated between the positive electrode and the negative electrode, and to a method for producing the same.
In general, alkaline storage batteries such as nickel-hydrogen batteries and nickel-cadmium batteries are produced by forming a group of electrodes by incorporating a separator between the positive electrode and the negative electrode, placing the group of electrodes inside a metallic outer casing (cell casing), welding a collector lead extended out from the positive electrode of the group of electrodes to the lower plane of the sealing body, and, after injecting the electrolyte, airtight sealing the cell casing by attaching the sealing body to the aperture portion of the cell casing. The separator used in the alkaline storage batteries above is required to have not only a sufficiently high mechanical strength, but also an excellent resistance against strong alkali included in the electrolyte, and it should retain a sufficient amount of electrolyte and maintain sufficiently high gas permeability while maintaining the alkaline electrolyte.
As a material for use in the separator above, mainly used conventionally was a non-woven cloth made of polyamide fibers; however, since this material suffered problems concerning its resistances against alkalis and oxidation, recently brought into use are non-woven cloth made of polyolefin resin fibers. In order to respond to the recent increase in capacity of alkaline storage batteries, however, there has been proposed to increase the cell capacity by reducing the volume of the separator by using thinner separators, thereby increasing the filling amount of active material. In this context, the use of fine-pored resin membrane separator has been proposed, because it enables a separator thinner than that made of a non-woven cloth.
The pore diameter of the fine pores of a fine-pored resin membrane separator is set small to prevent internal short circuit from occurring. However, the gas permeability decreases with reducing pore diameter of the fine pores. Thus, it has been proposed to impart hydrophilic properties or gas permeability to the fine-pored resin membrane separator. For instance, in JP-A-05-205719 is proposed to pattern forming a hydrophilic portion and a hydrophobic portion (non-hydrophilic portion) on the fine-pored resin membrane separator by means of printing.
Since the method proposed in JP-A05-205719 comprises pattern forming a hydrophilic portion and a hydrophobic (non-hydrophilic) portion by means of printing, not only the production method is simplified, but also the gas permeability is increased as to suppress the increase of internal pressure of the cell. In this manner, an alkaline storage battery with long life can be realized.
However, since the method proposed in JP-A-05-205719 comprises pattern forming the hydrophilic portion and the non-hydrophilic portion, there was limit in forming dense patterns, and it was found impossible to form hydrophilic regions and non-hydrophilic regions in fine units. Accordingly, it was found impossible to achieve ion permeability well compatible with gas permeability, and it resulted in problem of realizing sufficiently high cell characteristics.
In case a pattern of hydrophilic region and a hydrophobic region is formed on the fine-pored resin membrane separator, the portion of the hydrophilic region suffers poor gas permeability, whereas the portion of the non-hydrophilic region exhibits low ion permeability. In case it is impossible to form hydrophilic regions and non-hydrophilic regions in fine units, the gas generated in the portion of the hydrophilic region can no longer reach to the non-hydrophilic region as to increase the pressure inside the cells. In the non-hydrophilic region, on the other hand, difficulty is found on the permeation of ions, and the active material disposed opposed to this portion becomes less reactive as to lead to a drop in discharge characteristics.
In the light of the aforementioned circumstances, the invention has been made to overcome the problems above, and an object of the invention is to provide an alkaline storage battery having improved cycle life characteristics by suppressing the increase in internal pressure of the cell and also improved in utilization factor of the active material. Thus, there is provided an alkaline storage battery comprising a separator having densely and uniformly formed over the entire body thereof hydrophilic portions and non-hydrophilic portions, thereby providing a fine-pored resin membrane separator having excellent ion permeability and gas permeability.
In order to solve the problems above, the separator for use in the alkaline storage battery of the invention made from a polyolefin resin comprises densely and uniformly formed over the entire separator, first fine paths (series of pores) entrained from the surface to the back plane, and second fine path (series of pores) entrained from the back plane to the surface; provided that the inner wall of the first fine paths (pores) is imparted hydrophilic, and that the inner wall of the second fine paths (pores) is not imparted hydrophilic.
By thus forming first fine pores entrained from the surface to the backplane with the inner walls of the first pores rendered hydrophilic and second fine pores entrained from the back plane to the surface with the inner wall of the second fine pores rendered non-hydrophilic, the gas generated in the vicinity of the first fine pores having its inner walls rendered hydrophilic can be immediately reach the second fine pores provided in the vicinity of the second fine pores having its inner wall not rendered hydrophilic. Accordingly, the gas generated in the vicinity of the first fine pores can pass through the second fine pores to suppress the increase of the inner pressure. On the other hand, the ions passing through the second fine pores can immediately reach the first fine pores having the inner wall rendered hydrophilic. In this manner, the ions passing thorough the second fine pores are transferred through the first fine pores; hence, the utilization factor of the active material is increased because the active material disposed opposed to this portion is allowed to react more easily.
In case the volume ratio of the first fine pores with respect to the second fine pores is increased as large as 95:5, it has been found through experiments that the internal pressure of the cell increases and that the utilization factor of the active material decreases. Moreover, in case the volume ratio of the first fine pores with respect to the second fine pores should be decreased to 45:55, it has also been found that the internal pressure of the cell increases while decreasing the utilization factor of the active material.
From the results thus obtained, it can be understood that the volume ratio of the first fine pores and the second pores is preferably in a range of from 90:10 to 50:50.
To solve the aforementioned problems, the method for producing the separator for use in the alkaline storage battery of the invention made from a polyolefin resin comprises a mixing step comprising mixing a first powder comprising an alkali-soluble powder surface coated with a hydrophilic material, a second powder comprising an alkali-soluble powder, and a third powder comprising a polyolefin resin powder; a filming step for forming a membrane from the mixed powder; and an eluting step comprising immersing the membrane thus formed in an alkaline solution to elute out the alkali-soluble powder.
In case a membrane is formed by filming the mixed powder above, the resulting membrane comprises a portion made of continuously connected first powder made from the alkali-soluble powder coated with a hydrophilic material and a portion made of continuously connected second powder composed of the alkali-soluble powder finely and densely dispersed and arranged in the third powder made of polyolefin resin powder. Then, by immersing the resulting membrane in an alkaline aqueous solution to elute out the alkali-soluble powder, the hydrophilic material remains without being eluted out in the alkaline aqueous solution. Furthermore, the first powder is eluted out into the alkaline aqueous solution from the portion comprising the continuously connected first powder to provide an entrained eluted out portion, while the second powder is eluted out into the alkaline aqueous solution from the portion comprising the continuously connected second powder to provide an entrained eluted out portion.
In this manner, the first fine pores rendered hydrophilic and the second fine pores rendered non-hydrophilic are formed, while the first and the second fine pores are densely and uniformly dispersed inside the separator. Thus, there can be implemented an alkaline storage battery having its increase in internal pressure of the cell being suppressed, and having the utilization factor of the active material improved. To set the volume ratio of the first fine pores with respect to the second fine pores in a range of from 90:10 to 50:50 in this case, the mass ratio of the first powder and the second powder must be set in a range of from 90:10 to 50:50. Preferably, silicate powder is used as the alkali-soluble powder.